A liquid crystal is an ordered fluid of a class called nematic. Nematic fluids used for liquid crystal displays are made up of cigar-shaped organic molecules which are aligned in the same direction.
The display is constructed from two pieces of glass coated on the inner surfaces with transparent indium oxide conductors. On the first glass inner surface the conductive coating is shaped to the individual segments of the display (each segment being terminated to an individual contact pad). The second glass inner surface is a single conductive coating shaped to be common to all segments. This common conductor is called the backplane. The inner glass surfaces are also specially treated to align the molecules of the nematic fluid in a direction parallel to the plane of the glass. The first and second glass surfaces are further treated so that the molecules on the first glass inner surface are aligned 90.degree. from the molecules on the second glass inner surface. This alignment causes a twist in the molecular layers from one surface to the other. Displays constructed this way are called twisted Nematic Liquid Crystal Displays and are the most common.
The plane of polarization of polarized light will follow this twist and will exit the cell rotated 90.degree.. If the cell is put between crossed polarizers the cell appears clear. By applying an electrical field across the cell the twist is eliminated, since the molecules align themselves parallel to the electric field, and the cell appears dark. If the cell were put between parallel polarizers the cell would appear dark with no electrical field applied and light with a field applied. It is important that only an AC field be applied since a DC field causes electrolysis and deterioration of the electrodes.
Presently, each display segment is driven by a circuit including an exclusive OR gate connected to the segment electrode. A square wave is provided to the backplane and to one input of the exclusive OR gate; the other input of the gate is controlled by segment decoders. A logic "0" input produces a square wave output in phase with the input producing zero volts across the cell while a logic "1" inverts the square wave (180.degree. inversion) producing an RMS voltage equal to 1/2 the peak-to-peak voltages. Commercially available integrated circuit chips, e.g., the ICL 7136 available from Intersil, include circuits for driving numeral display segments but lack annunciator drive circuits. For most applications, annunciators such as decimal points, low battery indication, or identifiers such as volt or ohm indicators for meters must be controlled by additional external logic including the exclusive OR gate arrangement discussed above. One such logic chip is the CD 4076, a standard CMOS integrated circuit, available from many sources.